Zeolites are microporous crystalline materials formed by a framework of TO4 tetrahedra sharing all their vertices and giving rise to three-dimensional structures containing channels and/or cavities of molecular dimensions. They are of variable composition and T generally represents atoms with a formal oxidation state of +3 or +4, such as Si, Ge, Ti, Al, B, Ga, and the like. When any of the T atoms has an oxidation state of less than +4, the crystalline framework formed presents negative charges that are compensated by the presence of organic or inorganic cations in the channels or cavities. These channels and cavities can also hold organic molecules and H2O, so the resulting chemical composition of the zeolites can be represented by empirical formula:x(M1/nXO2):yYO2:zR:wH2Owhere M is one or more organic or inorganic cations with a charge +n; X is one or more trivalent elements; Y is one or more tetravalent elements, generally Si; and R is one or more organic cations. Although the nature of M, X, Y and R and the values of x, y, z, y and w can be modified by post-synthesis treatments, the chemical composition of a zeolite (as it is synthesised or after heating) has a range that is characteristic of each zeolite and its method of preparation.
The crystalline structure of each zeolite, with a specific system of channels and cavities gives rise to a characteristic X-ray diffraction pattern, which enables differentiation between them.
Many zeolites have been synthesised in the presence of an organic molecule that acts as a directing agent for the structure. The organic molecules that act as structure directing agents (SDA) generally contain nitrogen in their composition and can give rise to stable organic cations in the reaction medium.
The mobilisation of the precursor species during synthesis of zeolites can be carried out in the presence of hydroxyl groups and an alkaline medium, which can be introduced as the hydroxide of the same SDA, such as, for example, tetrapropylammonium hydroxide in the case of zeolite ZSM-5. Fluoride ions can also act as mobilising agents in the synthesis of zeolites. For example, in the patent EP-A-337479, the use of HF in water at low pH is disclosed as a mobilising agent of silica for the synthesis of zeolite ZSM-5.